To go to bed and fall asleep.
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When someone hits the sack, they're done with their day and ready to rest — usually after feeling tired or exhausted.
It's a casual, friendly phrase used in everyday conversation, more common in informal settings than in professional ones.
"Call It a Day"
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Stop an activity for the day, not necessarily to sleep
"Catch Some Z's"
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Take a short nap, not necessarily for the night
"Hit the Sack"
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Go to bed and sleep for the night